What causes the water in a swimming pool to turn green?

This requires understanding the specific circumstances in order to analyze it; what can be figured out just by guessing?

The following text was copied from the Internet: “The green coloration is caused by the growth of algae, just like in Taihu Lake. It is recommended to take the following measures: 1. Prevent the growth and reproduction of algae. Algae are extremely small plants, with hundreds of different types.” Algae can reproduce rapidly in water; they first consume the carbon dioxide dissolved in the water, causing the pH to rise quickly. As the dead algae then consume the oxygen in the water, a clear pool can turn into stagnant water within just one day. Spores in the air, as well as debris from lawns and soil, can enter water bodies and introduce algae; this often happens after heavy rains, especially when water temperatures are high. Generally, when there is sufficient residual chlorine in the water, algae growth can be effectively suppressed. The specialized algaecide, crystalline copper sulfate, is highly soluble in water; it inhibits the growth of algae and gives the water a blue color. It is worth noting that one should not take baths in home pools to avoid severely contaminating the water quality. 2 Controlling bacterial and viral contamination: To control harmful bacterial and viral contamination, disinfectants must be added to the pool water in an appropriate manner. For home users, the most commonly used disinfectants are bromine and its compounds, as well as chlorine and its compounds. Like chlorine, bromine can kill bacteria, viruses, and algae, and it also has an oxidizing effect. Elemental bromine is a reddish-brown liquid at room temperature; what is used for swimming pool disinfection are granular methyl compounds containing both bromine and chlorine. Adding this compound to the circulatory system results in the decomposition of hypobromous acid and hypochlorous acid. Bromine disinfection is more expensive than chlorine disinfection, but it operates over a wider pH range. Under soft water conditions, bromine is suitable as a disinfectant, with its concentration to be maintained at 4–6 mg/L. Hydrogen hypochlorite is a major disinfectant, and its mechanism of action is: ① Penetrating the cell walls of microorganisms and destroying their protein and enzyme systems ； ②Eliminate organic and inorganic pollutants in water through oxidation. The sources of hypochlorous acid include calcium hypochlorite or chlorine gas. Calcium hypochlorite is a granular substance formed when calcium hydroxide absorbs chlorine gas, and it is a stable compound. Calcium hypochlorite releases chlorine when added to water, and therefore can be used as a disinfectant for swimming pools. Adding calcium hypochlorite increases the concentration of calcium ions and the pH in water, which is beneficial for swimming pools that use soft water. Sufficient chlorine should be added to maintain residual chlorine in the pool water at 0.4–0.6 mg/L. When the organic matter concentration in the pool water is high, a mixture of NH2Cl, NHCl2, and NCl3 appears. This is produced by the reaction of chlorine molecules with nitrogen-containing compounds in the pool water. This eliminates the disinfecting effect, and in particular, NCl3 causes the pool water to emit an unpleasant odor. Only by increasing the chlorine dosage further can NCl3 be decomposed; at this point, the chlorine concentration in water stops rising, and it will only increase again after NCl3 has been decomposed – this is what is professionally referred to as breakpoint chlorination. When a swimming pool fails to receive sufficient fresh water for an extended period, the level of cyanuric acid in the water becomes too high; moreover, chlorination becomes ineffective due to the phenomenon of chlorine locking, in which chlorine exists primarily in the form of clusters of 2 or 3 chlorine molecules. It has not yet been determined at what concentration of cyanuric acid this phenomenon occurs; generally, it is believed that the chlorination locking effect is most pronounced at a concentration of around 160 mg/L. Once chlorination locking occurs in the pool water, increasing the chlorine dosage can alleviate this situation, but the long-term solution is to add more fresh water. On the other hand, cyanuric acid can act as a chlorine stabilizer; it is advantageous for it to remain at low concentrations in water, as this prevents the ultraviolet rays in sunlight from depleting the residual chlorine in the water. It is usually added to water in the form of particulates, and it can also be generated during the dissolution of dichloro complexes or trichloro complexes in water: the chlorine complexes split into individual chlorine molecules, while cyanuric acid is produced simultaneously. Therefore, if a chlorine complex is added to the pool, there is no need to add a separate chlorine stabilizer. However, when sodium hypochlorite or calcium hypochlorite is used as a disinfectant, a chlorine stabilizer must be added. Care should be taken to avoid mixing the aforementioned substances in a dry state, as that can be dangerous. Under special circumstances, an excess amount of sodium hypochlorite or calcium hypochlorite can be added to the pool water at once to create a high chlorine concentration, which suppresses algae growth and reduces the unpleasant chloroform odor; it can also break down organic pollutants that have gathered together. Furthermore, introducing an excessive amount of chlorine at once to raise the chlorine concentration in the pool water to around 10 mg/L can effectively prevent the growth of bacteria and algae; home pools can be treated in this manner once a week. 3 Maintain the clarity and cleanliness of the pool water. The clarity and cleanliness of water can be indicated by the total amount of soluble solids (TDS). TDS refers to charged chemical substances that accumulate in pool water, especially when the water evaporates or when not enough fresh water is added. This substance is difficult to see with the naked eye, but its conductivity causes corrosion of various components of the swimming pool, such as pumps, pipes, filters, etc. These substances usually exist in the form of chlorides or sulfates: long-term use of sodium hypochlorite as a disinfectant leads to the accumulation of chlorides ； Regular addition of substances such as aluminum sulfate and sodium bisulfate can lead to an increase in sulfate concentration. Periodic backwashing and timely addition of fresh water are the best ways to control TDS. 4 Stabilizing the pH of pool water: pH reflects the concentration of H+ or OH- ions in the pool water. The pH range for pool water as specified in the \"Hygienic Standards for Water Quality in Artificial Swimming Pools\" is 6.5 to 8.5. To ensure the proper functioning of the pool, its users should pay close attention to whether this parameter is within the acceptable range; the pH of the water can be measured using simple test strips. To stabilize the pH of the pool water, specialized stabilizers should be added to keep its pH within a reasonable range. At the same time, another indicator closely related to pH should be taken into account: total alkalinity. Total alkalinity reflects the ease with which the pH of water bodies changes. If the total alkalinity is below 80 mg/L, the stability of pH is insufficient, and it tends to fluctuate ； When the total alkalinity is higher than 200 mg/L, the pH is too stable to be adjusted. The pH of the pool water should maintain reasonable stability, allowing for adjustments without excessive fluctuations; therefore, the total alkalinity should be kept within a suitable range. Furthermore, high alkalinity and high pH can cause water to become turbid and lead to the formation of sediments ； Too low an alkalinity can cause corrosion in the equipment and lead to discomfort for swimmers. 5 Adjust the water hardness properly. The hardness of the water should be moderate; if it is too high, precipitates will form on the walls of the pool, while if it is too low, it will erode the mortar in those walls. Hard water refers to water in which the calcium ion concentration exceeds 250 mg/L and the alkalinity is higher than 150 mg/L. The pH in hard water remains relatively stable, but raising the water hardness should not be used to stabilize the pH. For hard water sources, a water softener should be installed at the water supply inlet for pre-treatment. Soft water has a calcium carbonate concentration of less than 50 mg/L or a calcium chloride concentration of less than 30 mg/L. It is difficult to maintain a stable pH in soft water. The use of calcium hypochlorite as a disinfectant helps to increase water hardness. There are mainly two methods to maintain clean water quality: one is to use a chemical dosing pump for automatic addition of chemicals, and the other is manual application of chemicals. If there are sediments present, they must be removed from the bottom of the water promptly until the water quality meets the standards. ”

It is said online that using chlorine together with hydrogen peroxide was a mistake, which caused the chemical to react on its own and lose its effectiveness.

The sterilization effect is poor; algae proliferate in hot weather, leading to an increase in COD

I don’t think algae are a problem; the residual chlorine level in swimming pool water is kept at 0.4–0.6 ppm, and this level is more than sufficient to prevent the growth of bacteria and algae.

Just like in an aquarium, algae can grow too rapidly, and high temperatures and strong light can contribute to this issue.

I thought it was an athlete.....